There have been proposed a number of active suspension systems in which the vertical relative displacement between a wheel and a vehicle body is controlled by using an actively controlled actuator according to the conditions of the road surface and the vehicle motion. According to such an active suspension system, a force proportional to the displacement corresponding to a spring force, and a force proportional to the rate of the displacement corresponding to a damping force can be freely generated with an hydraulic actuator according to the behavior of the vehicle body and the motion of the wheels, and it is possible to obtain both a favorable ride and a stable maneuverability.
Normally, the unsprung mass of a vehicle which typically consists of a wheel carrier, a wheel carried thereby, a tire, and associated brake and other power transmission elements has a resonance frequency of approximately 10 Hz, and the tire tends to lose its ability to maintain its contact with the road surface in this frequency range. However, the conventional control method for active suspension systems which merely replaces the forces generated by a spring and a damper with a force generated by an hydraulic actuator is unable to reduce the large oscillation amplitude of the unsprung mass in this frequency range, and it therefore makes little or no contribution in improving the ability of the tire to maintain its contact with the road surface in the frequency range around 10 Hz.